The photoconductivity of semiconductors has been known for approximately half a century. In this effect, free charge carriers are generated in a semiconductor by irradiation of light. The precise details of this process are material-dependent. Furthermore, even the smallest irregularities in the crystal structure of a semiconductor may lead to huge differences in terms of the number and the electrical properties, such as mobility and lifetime, of the charge carriers that are generated Inhomogeneities of this type have a negative effect on the quality of semiconductor components such as wafers. It is therefore of greatest interest to provide a detailed topography of the electrical properties of a wafer to detect crystal defects, particularly as early as during the production of the wafer. This may take place on the basis of the interaction of the optically generated charge carriers with an external microwave field.
DE 44 00 097 A1 describes a method in which the surface of a semiconductor material to be measured is exposed to light which excites charge carriers on the thin surface layer of the semiconductor material. An electromagnetic wave projected onto the surface of the semiconductor material is reflected by the surface to an extent which varies depending on the current charge carrier density. The reflected wave is detected and evaluated by a signal processing circuit.
The prior-art methods have the disadvantage of a relatively low detection sensitivity, with the result that they require unrealistically high injection rates for the use of the semiconductor components and need a long time to determine the detailed topography of an entire wafer.